ABSTRACT: Six cruises were carried out in the lower St. Lawrence Estuary (LSLE) and the NW Gulf of St. Lawrence (NW GSL) in spring and fall 1998, 2000, and 2001 to study the species composition, abundance, and distribution of macrozooplankton in relation to the physical environment. Our results confirm that the LSLE and the NW GSL represent 2 different physical environments. These differences are likely due to different circulation patterns observed between the 2 regions: the estuarine circulation in the LSLE and a quasi-permanent cyclonic gyre in the NW GSL. The dominant species found in both environments (LSLE and NW GSL) is the mysid Boreomysis arctica, but we observed no significant regional and interannual variations in its abundance. In contrast, 2 distinct groups characterized the LSLE and the NW GSL when we examined the other macrozooplankton groups. Two euphausiid species, Meganyctiphanes norvegica and Thysanoessa raschii, dominated in the LSLE. Their abundances were 6 and 15 times higher in the LSLE than in the NW GSL, respectively. On the other hand, the NW GSL was dominated by chaetognaths, hyperiid amphipods, and siphonophores. These groups were twice as abundant in the NW GSL as in the LSLE. Such inter-regional variations were attributed to different circulation patterns and different trophic systems. Furthermore, important interannual variations in the abundance of the major macrozooplankton species were also observed between 1998 and 2001 in the LSLE and the NW GSL. In the NW GSL, the arctic and boreo-arctic species were more abundant in 1998 than in 2000 and 2001. In contrast, their abundance was lowest in 1998, and highest in 2000 and 2001 in the LSLE. We hypothesize that stronger inflow of Labrador Shelf waters in the GSL via the Strait of Belle Isle may increase the advection of macrozooplankton into the LSLE.